1. Field of the Invention
The present invention relates to a glow plug degradation determination device that determines degradation of a glow plug of an internal combustion engine.
2. Description of Related Art
As a device for detecting a breakage abnormality in a glow plug of an internal combustion engine, there is a known device that senses voltage upstream of a glow plug and voltage downstream of the glow plug and determines a breakage in the glow plug based on a difference between the sensed voltages, e.g., as described in Patent document 1: JP-A-H11-182400. There is also a known device that has a dedicated circuit for detecting a breakage of a glow plug and that detects the breakage of the glow plug by comparing an electric potential in a series circuit including the glow plug with a reference electric potential corresponding to voltage of a power supply, e.g., as described in Patent document 2: JP-A-2002-276524.
For example, as shown in FIG. 15, a glow plug having a ceramic heater has a tip end, in which a U-shaped ceramic resistive element 100 is placed in a ceramic base 102. The ceramic resistive element 100 is formed of a conductive ceramic, which is made by bonding tungsten carbide (WC) grains with a grain boundary material (Si—Yb—O—N) as a grain boundary glass layer, for example. The ceramic base 102 is formed of an insulating ceramic, which is made by bonding grains including silicon nitride (Si3N4) as a main ingredient with a grain boundary material (Si—Yb—O—N) as a grain boundary glass layer, for example.
As the glow plug is used for a longer period, the grain boundary material Yb+ on a positive electrode side is diffused to a negative electrode side by a migration effect due to a heat load in the ceramic resistive element 100. As a result, fixing strength of the tungsten carbide (WC) is lowered and the positive electrode side of the ceramic resistive element 100 becomes porous, thereby increasing resistance of the ceramic resistive element 100.
As shown in FIG. 16, the heater temperature THG of the glow plug decreases if the resistance value R of the glow plug increases. In such the case, preheating of an internal combustion engine cannot be performed sufficiently, causing increase of hydrocarbon (HC) and carbon monoxide (CO) in exhaust gas. In a glow plug having a metal heater, the resistance is lowered by degradation due to a heat load.
Conventional devices can detect a breakage of a glow plug but cannot accurately detect degradation of the glow plug, which causes change in resistance of the glow plug. That is, since the detection is performed during an operation of the internal combustion engine, the glow plug is cooled by an intake air, an exhaust gas or a fuel spray. Therefore, it has been difficult to accurately detect the degradation even if the resistance is sensed during the operation of the internal combustion engine.